1. Field of the Invention
The present invention relates to the treatment of a wide variety of wounds. More particularly, the present invention relates to a method and pharmaceutical composition for not only accelerating wound healing but also preventing scar formation in skin, mucosa, and cornea.
2. Description of the Related Art
The primary function of the skin, mucosa and cornea is to serve as a protective barrier against the environment. Loss of the integrity of large portions or large wound of the skin, mucosa, or cornea as a result of injury or illness may lead to major disability or even death. Complications are a constant risk with wounds that have not fully healed and remain open for extended periods of time. On the other hand, over-repair with excess collagen accumulation will cause cosmetic problems and decreased tensile strength. Thus, it would be beneficial to accelerate the wound healing process but not to exaggerate the tissue remodeling response. However, present methods for both promoting wound healing and preventing scar formation are inadequate or ineffective.
Wound healing is a dynamic, interactive process involving soluble mediators, blood cells, extracellular matrix, and parenchymal cells. Wound healing has three phases, inflammation, tissue formation, and tissue remodeling. These phases overlap in time and are spatially triggered by many cytokines and growth factors differentially secreted from the recruited blood cells and parenchymal cells around the wound space (Clark, R. A. F. The molecular and cellular biology of wound repair. 2nd ed. New York:plenum Press, 1996).
In the first wound healing phase (inflammation), tissue injury causes the disruption of blood vessels and extravasation of blood constituents, which activate the clotting cascade and lead to hemostasis. The platelets forming clots release a number of vasoactive compounds and growth factors such as PDGF that attracts and activates macrophages and fibroblasts. Adherence of activated macrophages to the extracellular matrix further releases other chemoattractants such as GM-CSF, and TNF-α to recruit inflammatory leukocytes to the sites of injury. Infiltrating neutrophils cleanse the wound area of foreign particles and bacteria and are then extruded with the eschar or phagocytosed by macrophages.
In the second healing phase (tissue formation) beginning in the wound site three to four days after the injury, activated macrophages digest devitalized collagen and the fibrin clot. Dissolution of the clot allows the movement of more macrophages, fibroblasts and blood vessels to form the granulation tissue. Local release of growth factors such as EGF, TGF-α, and KGF from fibroblasts, macrophages, platelets, and epidermal cells stimulate the process. The activated macrophages also provide a continuing source of growth factors such as TGF-β, and VEGF necessary to further stimulate fibroplasias and angiogenesis. Thus, the stage is marked by not only the formation of new blood vessels to sustain the newly formed granulation tissue but also the proliferation of fibroblasts and their migration into the wound site where they produce an extracellular matrix, known as ground substance, comprised of collagen, fibronectin, and hyaluronic acid to replace the digested clot. On the other hand, re-epithelialization occurs during this stage of wound healing. Epithelial cells proliferate at the wound edges and migrate across the ground substance that serves as a scaffold upon which endothelial cells, fibroblasts and macrophages are also able to move. Migration is then halted by contact inhibition among epithelial cells, which at this point divide and differentiate to reconstitute the epithelium. In addition to re-epithelialization, the myofibroblasts derived through the differentiation of resident fibroblasts after TGF-β stimulation also use the newly deposited extracellular matrix to contract and promote more rapid wound closure. The contraction probably requires stimulation by TGF-β1 or TGF-β2 and PDGF.
In the third healing phase (tissue remodeling), collagen and matrix remodeling begin when granulation tissue formation begins and continues long after the wound has been covered by new epithelium and can continue for more than 1 year. This final stage of wound healing is characterized by devascularization and the replacement of granulation tissue and cells with a matrix comprised predominantly of type I collagen. Collagen remodeling during the transition from granulation tissue to scar is dependent on the continued synthesis and catabolism of collagen, and on the continued TGF-β stimulation. The degradation of collagen in the wound is controlled by several proteolytic enzymes termed matrix metalloproteinase, which are secreted by macrophages, epidermal cell, and endothelial cells, as well as fibroblasts. The various phases of wound repair rely on distinct combinations of matrix metalloproteinases, tissue inhibitors of metalloproteinase, and cytokines and growth factors.
In the wound healing processes, a variety of growth factors (such as EGF, FGF, KGF, PDGF, GM-CSF, TGF-α, TGF-β1, TGF-β2, TGF-β3, TNF-α, VEGF, IGF, IL-1) play pivotal roles in the transition from inflammation, tissue formation to tissue remodeling. However, the overall clinical experience with growth factors to accelerate wound healing has been discouraging. This is not surprising, considering that wound repair needs combinations of various growth factor stimulations, and a complex set of interactions among growth factors, blood elements, extracellular matrix, and cells. On the other hand, over-expression of TGF-β1, TGF-β2, IGF, or IL-1 may cause over-repair and excess accumulation of collagen within the wound, which results in scar formation and fibrosis, for example, hypertrophic scars, keloids, and radiation-induced fibrosis. Thus, in order to accelerate wound healing and to prevent scar formation or over-repair, it is necessary to stimulate the host to produce a variety of cytokines and growth factors in the early wound healing processes (inflammation and tissue formation) but to suppress the stimulation of cytokines and growth factors in the late wound healing process (tumor remodeling).
The primary goals of the treatment of wounds are rapid wound closure and a functional and aesthetically satisfactory scar.